1. Field of the Invention
The present invention relates to transmission lines in which signal conductors are closely arranged and the signal conductors transmit a plurality of high frequency signals of different types.
2. Description of the Related Art
In prior art, various types of transmission lines are developed for transmitting high frequency signals. For example, a transmission line described in Japanese Patent No. 4962660 has a stripline structure. The transmission line described in the Japanese Patent No. 4962660 includes an elongated dielectric base body, a signal conductor, a first ground conductor, and a second ground conductor. In the dielectric base body, the signal conductor is disposed at a midway location in a thickness direction. The first ground conductor and the second ground conductor are arranged so as to have the signal conductor between them in the thickness direction of the dielectric base body.
Furthermore, the first ground conductor and the second conductor are connected by a plurality of via-hole conductors (interlayer connection conductors) arranged along the signal conductor. This configuration provides a transmission line having a stripline structure in which the signal conductor is interposed between the first and second ground conductors.
When a plurality of transmission lines having the configuration described in Japanese Patent No. 4962660 is closely arranged inside a communication device and the like, it is conceivable, for example, to arrange the plurality of signal conductors on a single dielectric base body. In this arrangement, the plurality of signal conductors may be arranged along a direction perpendicular or substantially perpendicular to the thickness direction of the dielectric base body with space between the signal conductors.
In other words, it is conceivable to have a configuration in which the transmission lines each having the structure described in Japanese Patent No. 4962660 are placed side by side along the direction perpendicular or substantially perpendicular to the thickness direction of the dielectric base body.
However, along with downsizing of electronic devices to which transmission lines are installed, there is a demand for smaller transmission lines. On the other hand, in a case that neighboring signal conductors are placed too close, coupling occurs between these signal conductors. For example, in the case that the foregoing configuration is used, the distance between the signal conductors becomes shorter if the width of the transmission line is made narrower, causing the coupling to occur more easily between the signal conductors. As a result, isolation between the transmission lines each having such signal conductors decreases.